1. Field of the Invention
This invention relates to liquid crystal devices, and particularly to devices using liquid crystal having spontaneous polarization such as ferroelectric or antiferroelectric liquid crystal materials.
2. Description of Related Art
In recent years, the research on liquid crystal has rapidly progressed, resulting in increase of its application to devices, such as a display panel, an optical modulator, and an optical shutter in printing machine and so on.
In particular, liquid crystal devices and liquid crystal panels are characterized by thin depth, a lightweight, and low consumption. Hence, the devices or the panels are used as a display unit in various kind of devices, such as mobile terminals, for example cellular phones and mobile computers, moreover desk-top computers or household television sets.
The configuration of a liquid crystal display panel generally has a pair of opposing substrates spaced appropriately apart, including electrodes on inner faces of these substrates for switching each picture element or pixel defined by the arrangement of electrodes. Liquid crystal material is filled in a space between these substrates which are sealed at their periphery, while the detailed configurations are described later.
The widely used liquid crystal materials used these days for the liquid crystal display devices are; super twisted nematic liquid crystal and twisted nematic liquid crystal, which are hereinafter referred to as STN and TN respectively. The liquid crystal display of STN driven through a simple-matrix-type electrode configuration, which is referred to as simple matrix, is liable to generate image degradation caused by electrical cross-talk between picture elements or pixels, while the relatively low manufacturing cost results from the use of the simple matrix. The liquid crystal display of STN driven through the simple matrix also has undesirable response time (slow response time) for displaying dynamic images such as moving pictures.
On the other hand, TN in devices may be driven through an active-matrix-type electrode configuration including thin film transistors as switching elements, where the configuration is referred to as active matrix.
The liquid crystal display device of TN driven through the active matrix generally has no problem of electrical cross-talk, hence its image quality is better than that produced by the liquid crystal display device of STN driven through the simple matrix. The liquid crystal display device of TN further has faster response time than that of the liquid crystal display of STN, however the response time is limited by the characteristics of material itself, which means the liquid crystal display of TN being unsuitable for a display panel required to respond at high speed for displaying dynamic images such as motion picture.
These two materials have a common problem of a narrow view angle, hence the preferable view direction to see images on the display using these materials is limited.
It is well known that a certain type of liquid crystal material has spontaneous polarization, and a ferroelectric liquid crystal, which is referred to as FLC hereinafter, is a representative one of this type. This type of liquid crystal material is characterized by its fast response time ranging from several to several hundred microseconds, which is approximately one hundred times faster than that of TN liquid crystal. Therefore, this type of material may solve the problem related to the response time.
The FLC also has a characteristic such that liquid crystal molecules of the FLC always maintain their axis parallel to an appropriately treated surface of a substrate contacting the crystal. This characteristic leads to extremely smaller variation of index of refraction of the crystal according to view direction than that of TN or STN liquid crystals, resulting in a wider viewing angle of display panels using FLC material. Therefore the FLC material also has the advantage suitable for the liquid crystal as the material for a display panel.
However, the FLC material has a disadvantage such as a decrease of contrast ratio (or low contrast ratio) when used as the material for the display panel. The decrease comes from incompleteness of the memory effect during data being maintained in a picture element, where data corresponds with whether the light transmittance of the material in the element is low or high. That is, a little increase of the light transmittance in the picture element occurs when data pulses of zero amplitude for displaying “black” during several frames are applied to the element, and a little amount of light from a light source leaks through the element, resulting in decreasing the contrast ratio.
Therefore, the prevention of the increase of the light transmittance during displaying “black” is desired for improving the contrast ratio of the display panel using the FLC material.